1. Field of the Invention
This invention relates to the cleaning of well formations and of equipment used in the production and treatment of hydrocarbon fluids. More particularly, the invention is directed to methods involving particular compositions for cleaning and removing asphaltene-containing organic deposits from such surfaces. The invention is further directed to the use of such compositions to avoid or minimize the deposition of asphaltenes during well treatment operations and in the transport of hydrocarbon well fluids.
2. Discussion of the Prior Art
Asphaltenes are high molecular weight, complex aromatic ring structures containing O, N, S and heavy metals. They occur widely in heavy oil-producing formations, giving such crude oils their color; heavier, black-oil crudes will typically have higher asphaltene content. Asphaltenes also comprise a major component of native asphalts and bitumens.
The presence of asphaltenes in well fluids can cause severe difficulties when producing the well. Asphaltenes, being polar molecules, tend to bond to charged surfaces, especially clays, leading to formation plugging and to oil wetting of formations. In crudes, asphaltenes are usually present not in solution but colloidially dispersed, as a colloidal dispersion stabilized by oil resins. Incompatible liquids introduced during well treatments and workovers including acidizing, condensate treatments and the like may destabilize the asphaltene micelles, leading to asphaltene precipitation. Very high gas-liquid ratios as are encountered in CO2 floods or gas wells can also cause precipitation of asphaltenes.
Precipitated asphaltenes cause many problems at various stages of the well production and during processing of crude oils. In the field, precipitation causes filter plugging, gives rise to high viscosity well fluids that require high pumping pressure and difficult-to-treat emulsions, and deposits asphaltenes on tank bottoms and internal surfaces of equipment. As these deposits accumulate in well tubing, pipelines, production equipment and storage facilities, as well as on the faces of producing formations in oil and water wells, they block fluid flow and gradually decrease production until remedial work is required to remove them.
In processing operations, high velocity flow creates a charge that can exacerbate precipitation in valves and pipes and other conveying devices. On hot surfaces such as those found, for example, in heat exchangers, carbonization or coking of asphaltene deposits can make their removal very difficult. The deposits reduce the efficiency of plants and in the worst case can lead to a complete blockage and to a shut-down of production, which results in high costs.
Heavy oils, widely used alone or in mixtures with heavy distillates for powering ships and as furnace fuel in industrial plants and power plants, comprise considerable amounts of asphaltenes, resins and paraffinic waxes. Absent preventive measures, these often precipitate out, leading both to poor combustion and to difficulties during handling and storage of the fuel. Combustion disturbances due to the precipitation of asphaltenes are also observed in power stations operated with heavy oils.
The tubing and piping of gas-producing wells have also been observed to become plugged with organic deposits including paraffins as well as compositions comprising a variety of fused ring structures including those termed diamondoids.
Organic deposits derived from crude oil have thus long been a source of trouble and operating expense to petroleum producers and refiners, as well as to users of heavy oils as fuels. Paraffin and resin deposits commonly encountered in such operations are generally in the form of soft, low melting waxy materials that are removed with a variety of solvents and by mechanical means. Asphaltene deposits, however, are hard, amorphous, high melting, adherent materials that are difficult to remove by mechanical methods, and washing with conventional solvents or hot well fluids is relatively ineffective in their removal. Asphaltenes with their aromatic ring structure may be dissolved with aromatic solvents like xylene and toluene and in highly aromatic well fluids, but they are substantially insoluble in paraffinic well fluids and straight chain alkanes such as hexane, heptane and the like.
A variety of chemical treatments are disclosed in the art for removing asphaltenes including the use of solvents, combinations of dispersants and solvents, and mixtures comprising oil, dispersants and solvents. Diesel oil, by itself and in combination with certain additives, has been disclosed to dissolve asphaltenes. For example, the combination of diesel oil and 1% n-butylamine is disclosed to be substantially superior to diesel oil alone with respect to dissolving asphaltenes, but the combinations of diesel oil with aromatic hydrocarbons including xylene and toluene appear to provide very little, if any, improvement in solvent power over diesel oil alone.
The dispersant-plus-solvent approach has been disclosed for removing asphaltenes from formations, and a variety of suitable dispersant compositions are known and available to the trade for this purpose. Continuous treating may be required to inhibit asphaltene deposition in well tubing, while batch treatments are commonly used for cleaning dehydration equipment and tank bottoms. Asphaltene precipitation inhibitors have also been disclosed for use in continuous treatment or squeeze treatments of well formations.
Small amounts of dispersing agents may be effective to prevent or reduce the precipitating-out of asphaltenes, or to reduce the tendency of such precipitates to become deposited on surfaces. A variety of compounds suitable for use as asphaltene-dispersing agents are known, including dodecylbenzenesulfonic acid, alkane sulfonic acids, and alkoxylated amines, and their use in such applications is widely described in the art. Dispersants and emulsifiers employing mixtures of alkoxylated fatty amines and metallic soaps, and dispersants comprising alkylphenol-formaldehyde resins in combination with hydrophilic-lipophilic vinyl polymers are also known to be suitable for these purposes. However, since oils vary in their composition, individual dispersing agents can operate effectively only in a limited range, and even small changes in the oil composition can have a major effect on the dispersing properties for asphaltenes.
Although dispersants and precipitation inhibitors address the problem of slowing or preventing asphaltene precipitation, once asphaltene deposits form, the use of such compositions in their removal generally requires a shut down and loss of production. More effective methods and compositions for removing asphaltene deposits are thus clearly needed. In addition, adjuvants for asphaltene solvent compositions that enhance the effectiveness of asphaltene dispersants and precipitation inhibitors would represent a further valuable advance in the production and processing of heavy crudes and bitumens.
As noted above, while paraffinic wax deposits may be removed by washing with light distillates and other predominately paraffinic hydrocarbons, aromatic solvents are generally regarded as the solvent of choice for dissolving asphalts. Paraffinic hydrocarbons such as propane, hexane and the like are precipitants for asphalt, and are commonly used to extract oils from heavy, asphaltene-containing hydrocarbon fluids. The art has heretofore largely regarded olefins to be precipitants for asphaltenes, and for this and other reasons has avoided their use in well treatment. In Journal of Canadian Petroleum Technology, 1984, pp. 1-7, the researchers observed that “Olefins which are present in many crudes will cause precipitation of asphaltenes similar to paraffins. Olefinic hydrocarbons are undesirable as injection solvents, not only because of asphaltene precipitation but also due to their reactivity.” More recently, workers have explored the use of olefins in well treatment.
For example, published Russian patent applications 2,162,517; 2,166,624; and 2,178,070, published Jan. 27, 2001; May 10, 2001 and Jan. 10, 2002, respectively, disclose methods for removing asphaltenes from the bottom zone of wells producing heavy oils and dead oils. Russian patent application 2,162,517 discloses a method in which (a) a mixture of (i) alpha-olefins and (ii) light pyrolysis tar or products based on it in a volumetric ration of 9:1 to 1:9 and (b) an alkaline solution are introduced into and removed from the well in a repressive-depressive wave regime with a frequency of 1-400 Hz. Russian patent application 2,166,624 discloses a method in which an alpha-olefin fraction having a distillation temperature of 70-300° C. treated with hydrogen peroxide in a mole ratio of 0.1-0.5:1.0 of the alpha-olefin fraction is introduced into the well. Russian application 2,178,070 discloses the injection into the well of C6-C20 alpha-olefins with a distillation temperature of 70-3000.
However, these workers examined only the relative effectiveness of the fraction in removing an asphalt-tar-paraffin deposit from a test cell containing quartz sand, compared with a hydrocarbon prototype fluid. The modest improvement in permeability observed by these workers appears to be due only to a greater solubility of the paraffin and tar components of the deposit in the olefin fraction, which these workers demonstrated by extracting light pyrolysis tars with alpha-olefin fractions in similar tests, disclosed in the published Russian patent application 2,162,517.
In U.S. Pat. No. 5,674,816 there are disclosed frac fluids comprising linear olefins, including alpha-olefins, having 10 or more carbon atoms. The linear olefins may be used alone or in mixtures. Frac fluid compositions are said to be selected in part on the basis that they not precipitate asphaltenes. However, there is no suggestion that the compositions disclosed therein would be particularly effective as solvents for asphaltenes, or that they would be effective in removing organic deposits, particularly deposits comprising asphaltenes.
It is thus surprising that olefins in combination with kerosene or an aromatic solvent containing up to 16 carbon atoms may be useful for removing asphaltene deposits and organic deposits comprising such asphaltenes from surfaces. The art does not disclose or suggest that olefins, more particularly C4-C30 olefins in combination with kerosene or an aromatic solvent containing up to 16 carbon atoms, would be highly effective.